This invention relates in general to power supplies and in particular to output stages for switching regulators.
Power supplies are classified under linear regulators, switching regulators and resonant converters. Linear regulators are relatively inefficient at converting an input voltage to an output voltage, delivering up to 25 watts at 30-50% efficiency. Resonant converters are far more efficient, delivering up to 150 watts with an efficiency of 85-90%. At the higher wattages, however, switching regulators are preferred, delivering up to 2000 watts with an efficiency between 70 and 90%.
Switching regulators include buck regulators and flyback regulators. The buck regulator converts a high input voltage to a lower output voltage. A power output switch chops up the input voltage into a stream of pulses which are rectified into direct current by an output stage. Although low in cost, weight and size, the buck regulator does not provide isolation between the input and output. Further, it usually provides only one output.
The flyback regulator, in contrast, can provide multiple outputs per circuit with full isolation. A power input switch chops up the dc voltage across the primary winding of a main switching transformer. A series of output stages are inductively coupled to the primary winding, each rectifying and filtering the induced voltage to provide an output voltage.
A basic output stage 2 for a switching regulator is shown in FIG. 1. The output stage 2 includes a choke 4, a diode 6 and a capacitor 8, which form a main loop. Energy is stored in the choke 4. In a flyback regulator, the choke 4 is physically part of the main switching transformer. Through half-wave rectification, the diode 6 provides a direct current with large ripple. The ripple is greatly reduced by the capacitor 8, which stores charge during most of the period when the diode 6 is conducting and releases charge during the rest of the period. The voltage across the capacitor 8 is the output voltage V.sub.out.
However, a substantial increase in load will cause a significant increase in voltage drop through the diode 6 and any other resistance components. Thus, the output voltage will also change significantly.
Most integrated circuits do not function properly unless the output voltage is maintained within a certain range. For example, a five volt supply for bipolar logic usually must be regulated between 4.75 and 5.25 volts.
Supplies capable of such regulation are presently available. The output voltage is regulated by a control loop (usually in the input stage), typically including a reference, integrator, attenuator (if needed), loop stability compensator and feedback signal. The feedback signal is supplied to an integrator (which provides negative feedback for stabilization) where it is effectively compared with the reference signal. The output of the integrator is compared to an oscillatory signal (usually a triangular waveform) by a comparator. The comparator functions as a pulse width modulator and controls the power input switch duty cycle, which controls the amount of energy stored in choke 4.
However, modern power supplies cannot easily be regulated to meet the accuracy and bandwidth required by the newer integrated circuits which, due to closer spacing of internal components, operate at lower voltages. Integrated circuits rated at 3.3 volts are presently available, and technology forecasts are predicting that the newer integrated circuits will be operated between two and three volts.